1. Field of the Invention
The present invention is directed to a process for reducing the free monomer content in polyisocyanate adduct mixtures prepared from monomeric diisocyanates.
2. Description of the Prior Art
Polyisocyanate adducts are well known in polyurethane chemistry and include isocyanate-terminated prepolymers, biurets, allophanates, isocyanurates and the reaction products of diisocyanates and compounds containing two or more isocyanate-reactive hydrogens. During the production of many of these adducts it is common to use large excesses of the monomeric diisocyanates in order to minimize the formation of high molecular weight products. However, the presence of unreacted monomeric diisocyanates in the final product is undesirable due to the high vapor pressure of the monomeric diisocyanates. Accordingly, after preparation of the adduct mixtures the content of unreacted diisocyanates (free monomers) must be reduced in order to obtain products which have a low vapor pressure.
The initial reduction of the free monomer content is commonly accomplished in conventional distillation apparatus such as a falling film thin-layer evaporator. However, for the final stage of monomer reduction the use of agitated thin-layer or wiped-film evaporators is preferred since they help to maintain uniform temperatures throughout the apparatus. Localized high temperature areas should be avoided to the extent possible in order to minimize the additional reaction of the free monomers with the adducts and to prevent thermal degradation and discoloration. While these conventional distillation procedures can successfully reduce the free monomer content, a need exists for the further removal of these monomers from the polyisocyanate adduct mixture because of the previously discussed effect of their vapor pressure.
It is an object of the present invention to reduce the free monomer content of polyisocyanate adduct mixtures prepared from monomeric diisocyanates.
It is an additional object of this invention to reduce the free monomer content while minimizing discoloration and thermal degradation of the adduct mixtures and also minimizing any substantial further reaction of the free monomers with the polyisocyanate adducts.
It has surprisingly been found that these objects may be accomplished by the present invention which is discussed in more detail below.